1. Field of the Invention
The present invention relates to a vinyl alcohol polymer and a process for producing the same. In particular, it relates to a vinyl alcohol polymer having a high syndiotacticity and a high degree of polymerization which contains a vinyl alcohol unit and a vinyl ester unit, and also to a process for producing the same.
2. Description of the Prior Art
Polyvinyl alcohol, i.e., hydrolyzed product of polyvinyl acetate is one of the crystalline water-soluble polymers limited in number, and has excellent interfacial characteristics and mechanical properties, which fact leads to such uses as paper processing, stabilizers for emulsions and the like. In addition, it is widely known that they are used as raw materials for polyvinyl alcohol films and the like. Recently, the uses thereof in new fields such as raw materials for gels have positively been developed.
Polyvinyl alcohols commercially available heretofore are stereospecifically so-called "atactic" and have a diad syndiotacticity of about 53 mol %. As is commonly known, a polyvinyl alcohol with excellent stereoregularity has physical properties significantly different from those of atactic polymers due to hydrogen bond, and especially the polyvinyl alcohol having a diad syndiotacticity of at least 55 mol % becomes easy to crystallize, which can make the usefulness of the polyvinyl alcohol increase. For the purpose of obtaining a polyvinyl alcohol with a high syndiotacticity, there are proposed several processes, among which a process comprising the hydrolysis of polyvinyl pivalate can be mentioned. For example, Sakaguchi et al. reported that a polyvinyl alcohol was obtained by hydrolyzing polyvinyl pivalate in a mixed solvent of acetone and water in the presence of potassium hydroxide (cf. Kobunshi Kagaku, 27, 758-762 (1970)). Nozakura et al. reported that a polyvinyl alcohol was obtained by hydrolyzing polyvinyl pivalate in a mixed solvent of either acetone and methanol or dioxane and methanol, followed by the hydrolysis thereof in methanol in the presence of potassium hydroxide (cf. Journal of Polymer Science: Polymer Chemistry Edition, 11, 279-288 (1973)). Furthermore, Imai et al. reported that a polyvinyl alcohol was obtained by hydrolyzing polyvinyl pivalate in acetone and then in dimethyl sulfoxide in the presence of potassium hydroxide (cf. Journal of Polymer Science: Polymer Chemistry Edition, 26, 1961-1968 (1988)).
However, in general, since such a polyvinyl ester having a bulky side chain as polyvinyl pivalate is difficult to be hydrolyized due to its steric hindrance, highly hydrolyzed polyvinyl alcohol cannot be obtained under the conditions as applied conventionally to polyvinyl acetate. For example, according to the above-mentioned paper of Sakaguchi et al., the degree of hydrolysis of the obtained polyvinyl alcohol is confined below about 52%. On the other hand, according to the above-described paper of Nozakura et al., the degree of hydrolysis and the degree of polymerization of the obtained polyvinyl alcohol are unknown. The present inventors therefore made the supplementary test according to the process shown by Nozakura et al., and found that polyvinyl pivalate was hydrolyzed to give a polyvinyl alcohol having a rather high degree of hydrolysis, which colored dark brown and degraded. Moreover, this process is troublesome to conduct due to the need of repeated hydrolysis. Besides, according to the above paper of Imai et al., the obtained polyvinyl alcohol had a low degree of polymerization, which fact suggests that the degree of polymerization had decreased during the hydrolysis. To make matters worse, the process reported by Imai et al. is also troublesome to conduct due to the two-stage hydrolysis.